Soil decontamination, aimed at the destruction of harmful organisms in the soil, is often necessary in agriculture. These organisms, notably nematodes, fungi and bacteria, can have an undesirable effect both on the quality and on the quantity of the harvest. Soil decontamination can be carried out in various ways, namely by chemical, physical and biological means, heating being a physical method.
In the case of chemical soil decontamination, which is a conventional way of sterilizing the soil, chemicals that kill the harmful organisms are introduced into the soil. However, chemical soil decontamination has a number of major limitations, notably from the environmental point of view. Since chemicals can directly affect the living environment and/or the farm produce to be grown, their use for chemical soil decontamination is strictly regulated by the requirements for a permit and by the directions for use.
Methyl bromide, a colourless and almost odourless toxic gas, was widely employed for chemical soil decontamination in the past. However, methyl bromide attacks the ozone layer, and its use has been forbidden under the Montreal Protocol since 1 Jan. 2005. Its production should also stop as from 1 Jan. 2015. An alternative soil decontaminating agent is for example 1,3-dichloropropene, but the use of this chemical has also been forbidden in the European Union since March 2008. In the Netherlands, the use of methyl bromide for soil decontamination was made illegal as long ago as 1992. The only permissible alternative—Monam®—is of limited applicability and can only be used at most once every five years.
The use of a product of plant origin for soil decontamination has been described e.g. in WO 02/056683. In this case, allicin—a compound that is liberated when garlic is pressed—is applied to the soil before sowing or planting in order to protect the plants from pathogenic organisms, such as fungi, bacteria, protozoa and nematodes. A drawback of allicin is that it is a chemically unstable volatile compound, so that its large-scale use is impractical; in fact, allicin is most suitable for soil decontamination in small areas or in greenhouses.
In the case of physical soil decontamination, the harmful organisms present in the soil are killed by heating it to 80-90° C., for example by passing steam into the soil or heating the soil with flames. However, the use of steam has the disadvantage of being very costly for use on a large scale, while if flames are used, it is very difficult to achieve a uniform heating of the soil. Quite in general, physical soil decontamination is rather expensive and gives variable results. WO 03/099004 discloses a system and a method for the in situ sterilization of the soil and the destruction of insects and weeds in it. This method consists of exposing the soil down to a specified depth to an energy flux with a microwave frequency so chosen as to dissociate the biopolymers.
Biological soil decontamination (BSC) is an alternative to chemical and physical soil decontamination. In this case, fresh, readily degradable plants or parts of plants, such as grass cuttings are introduced into the soil, and the latter is hermetically covered with a plastic film for some time, generally for 6 weeks. It has been found that this reduces the number of pathogens, such as nematodes and fungi, in the soil. Due to the indirect action of the materials introduced into the soil in the case of biological soil decontamination, none of the plant protection legislation is applicable here.
The use of biological soil decontamination in asparagus cultivation has been described for example in Plant Life, 2 (2008), where the authors studied the combating of Fusarium oxysporum f. sp. asparagi and Fusarium redolens f. sp. asparagi. Although fusarium is not fully destroyed, the asparagus plants planted out in the decontaminated plots seemed to do better. The authors also reported a better quality and a higher yield of asparagus over several years after this biological soil decontamination.